The terms wide band and narrow band are often used when discussing FM radio signals. For several decades, wide band referred to FM broadcasting, which in the USA, occupies 150 kHz of bandwidth and has a channel spacing of 200 kHz. In contrast, narrow band referred to FM with a modulation index less than or equal to 1. However, these phrases have been used somewhat differently in recent years, because Congress and the FCC have mandated a reduction in the channel spacing of public safety FM radios, so the older requirement is sometimes called wide band and the new requirement called narrow band, even though both are narrow band according to the modulation index criteria.
Amateur radio operators are required to use a modulation index less than or equal to 1. (47 CFR Ch 1 Part 97 para. 97.307f1) Although national band plans do not specify channel spacing, typical channel spacing can be gleaned from the practice of repeater coordinators. For example, the Northern Amateur Relay Council of California specifies 20 kHz spacing below 146 Mhz, and 15 kHz spacing above, in the 2 meter (144-148 MHz) band.
Experiments with a Yaseau FT-90R shows this fairly modern tranceiver will reject a low power signal from a handheld radio at a channel spacing of 15 kHz at a distance of only one metre (when tuned to 146.45 MHz).
The relationship between modulation index is given on page 9-3 of the 1990 ARRL Handbook: Χ = D/m where Χ is the modulation index, D is the peak deviation (half the difference between the maximum and minimum values of the instantaneous frequency), and m is modulation frequency in hertz.
Applying the FCC rule requiring that Χ be no greater than 1, and
assuming the maximum modulating frequency is 3 kHz, we have
1 >= D/3000 or
D = 3000 Hz
This seems to be in conflict with the commonplace description of amateur FM signals having a deviation of 5 kHz, which would seem to indicate a modulation index of 5/3 = 1.7.
In FCC terminology the Public Safety Radio Service is one of the Private Land Mobile Radio Services, and is regulated by Part 90 of the rules and regulations. Paragraph 90.209 authorizes various channel spacings and bandwidths, depending on frequency.
From 150 to 174 MHz, the channel spacing is 7.5 kHz and the authorized bandwidth is 20/11.25/6 kHz. These are for stations authorized after 1995, and the authorized bandwidth depends on equipment design. After 1 January 2011, no new applications will be accepted for authorized bandwidth exceeding 11.25 kHz.
This frequency range is the same as 150-174 MHz except that the channel spacing is 6.25 kHz.
According to California Department of General Services, Telecommunications Division, "There'ss an FCC mandate lurking outside your communications center,and it's been around for many years. It says you have until January 1, 2013 to migrate from your wide band (25 kHz) radio systems to narrowband (12.5 kHz or less). This applies to licensees using the LMR spectrum below 512 MHz. If you operate channels in the 150 -174 MHz (VHF high-band), 450-470 and 470-512 MHz (UHF bands), this means you."
The relevant FCC order was released December 23, 2004. It puts a limitation on the relevant frequencies (one by one, in a table) that "this frequency will be assigned with an authorized bandwidth not to exceed 11.25 kHz. In the 450-470 MHz band, secondary telemetry operations pursuant to § 90.238(e) will be authorized on this frequency."